Capture One only adopts the latest process engine on newly imported images, it does not automatically upgrade the process engine on existing images.
The processing method or engine determines the way in which a RAW file is demosaiced, color managed and presented on screen. The tools and their adjustments in Capture One can produce different results depending on how they interact with the processing engines.
Although advances in engines can dramatically improve how the image is rendered, several legacy engines are included to support previously used workflows, so there is no real need to upgrade.
However, should you wish to take advantage of new tools and advances in the process engine with existing images, you can manually upgrade the engine on selected variants from the Base Characteristics tool.
NOTE: If you choose to upgrade the process engine, it is recommended that important images are cloned first so that they can not only be compared before committing but you can also retain a copy of the image with the adjustments.
As upgrading the engine on variants has an irreversible effect on adjustments, this is an especially important consideration if you have finalized the images for output and if those have already been submitted to your client.